Linux内核kprobe机制实现浅析

   Kprobe 机制是内核提供的一种调试机制，它提供了一种方法，能够在不修改现有代码的基础上，灵活的跟踪内核函数的执行。它的基本工作原理是：用户指定一个探测点，并把一个用户定义的处理函数关联到该探测点，当内核执行到该探测点时，相应的关联函数被执行，然后继续执行正常的代码路径。

     Kprobe提供了三种形式的探测点，一种是最基本的kprobe，能够在指定代码执行前、执行后进行探测，但此时不能访问被探测函数内的相关变量信息；一种是jprobe，用于探测某一函数的入口，并且能够访问对应的函数参数；一种是kretprobe，用于完成指定函数返回值的探测功能。其中最基本的就是kprobe机制，jprobe以及kretprobe的实现都依赖于kprobe，但其代码的实现都很巧妙，强烈建议每一个内核爱好者阅读。

    

    好了，闲话少叙，开始上代码：

  首先是struct kprobe结构，每一个探测点的基本结构。

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 struct kprobe {
     /*用于保存kprobe的全局hash表，以被探测的addr为key*/
     struct hlist_node hlist;

     /* list of kprobes for multi-handler support */
     /*当对同一个探测点存在多个探测函数时，所有的函数挂在这条链上*/
     struct list_head list;

     /*count the number of times this probe was temporarily disarmed */
     unsigned long nmissed;

     /* location of the probe point */
     /*被探测的目标地址*/
     kprobe_opcode_t *addr;

     /* Allow user to indicate symbol name of the probe point */
     /*symblo_name的存在，允许用户指定函数名而非确定的地址*/
     const char *symbol_name;

     /* Offset into the symbol */
     /*如果被探测点为函数内部某个指令，需要使用addr + offset的方式*/
     unsigned int offset;

     /* Called before addr is executed. */
     /*探测函数，在目标探测点执行之前调用*/
     kprobe_pre_handler_t pre_handler;

     /* Called after addr is executed, unless... */
     /*探测函数，在目标探测点执行之后调用*/
     kprobe_post_handler_t post_handler;

     /*
      * ... called if executing addr causes a fault (eg. page fault).
      * Return 1 if it handled fault, otherwise kernel will see it.
      */
     kprobe_fault_handler_t fault_handler;

     /*
      * ... called if breakpoint trap occurs in probe handler.
      * Return 1 if it handled break, otherwise kernel will see it.
      */
     kprobe_break_handler_t break_handler;

     /*opcode 以及 ainsn 用于保存被替换的指令码*/

     /* Saved opcode (which has been replaced with breakpoint) */
     kprobe_opcode_t opcode;

     /* copy of the original instruction */
     struct arch_specific_insn ainsn;

     /*
      * Indicates various status flags.
      * Protected by kprobe_mutex after this kprobe is registered.
      */
     u32 flags;
 };

    对于kprobe功能的实现主要利用了内核中的两个功能特性：异常（尤其是int 3），单步执行（EFLAGS中的TF标志）。

    大概的流程：

 1）在注册探测点的时候，对被探测函数的指令码进行替换，替换为int 3的指令码；

 2）在执行int 3的异常执行中，通过通知链的方式调用kprobe的异常处理函数；

 3）在kprobe的异常出来函数中，判断是否存在pre_handler钩子，存在则执行；

 4）执行完后，准备进入单步调试，通过设置EFLAGS中的TF标志位，并且把异常返回的地址修改为保存的原指令码；

 5）代码返回，执行原有指令，执行结束后触发单步异常；

 6）在单步异常的处理中，清除单步标志，执行post_handler流程，并最终返回；

     下面又进入代码时间，首先看一下kprobe模块的初始化代码，初始化代码主要做了两件事：标记出哪些代码是不能被探测的，这些代码属于kprobe实现的关键代码；注册通知链到die_notifier，用于接收异常通知。

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 初始化代码位于kernel/kprobes.c中
 static int __init init_kprobes(void)
 {
     int i, err = 0;
         ....

      /*kprobe_blacklist中保存的是kprobe实现的关键代码路径，这些函数不应该被kprobe探测*/
     /*
      * Lookup and populate the kprobe_blacklist.
      *
      * Unlike the kretprobe blacklist, we'll need to determine
      * the range of addresses that belong to the said functions,
      * since a kprobe need not necessarily be at the beginning
      * of a function.
      */
     for (kb = kprobe_blacklist; kb->name != NULL; kb++) {
         kprobe_lookup_name(kb->name, addr);
         if (!addr)
             continue;

         kb->start_addr = (unsigned long)addr;
         symbol_name = kallsyms_lookup(kb->start_addr,
                 &size, &offset, &modname, namebuf);
         if (!symbol_name)
             kb->range = 0;
         else
             kb->range = size;
     }
         ....
     if (!err)
         /*注册通知链到die_notifier，用于接收int 3的异常信息*/
         err = register_die_notifier(&kprobe_exceptions_nb);
          ....
 }
 其中的通知链：
 static struct notifier_block kprobe_exceptions_nb = {
     .notifier_call = kprobe_exceptions_notify,
     /*优先级最高，保证最先执行*/
     .priority = 0x7fffffff /* we need to be notified first */
 };

     kprobe的注册流程register_kprobe。

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 int __kprobes register_kprobe(struct kprobe *p)
 {
     int ret = 0;
     struct kprobe *old_p;
     struct module *probed_mod;
     kprobe_opcode_t *addr;

     /*获取被探测点的地址，指定了symbol_name，则从kallsyms中获取；指定了offset，则返回addr + offset*/
     addr = kprobe_addr(p);
     if (!addr)
         return -EINVAL;
     p->addr = addr;

     /*判断同一个kprobe是否被重复注册*/
     ret = check_kprobe_rereg(p);
     if (ret)
         return ret;

     jump_label_lock();
     preempt_disable();
     /*判断被注册的函数是否位于内核的代码段内，或位于不能探测的kprobe实现路径中*/
     if (!kernel_text_address((unsigned long) p->addr) ||
      in_kprobes_functions((unsigned long) p->addr) ||
      ftrace_text_reserved(p->addr, p->addr) ||
      jump_label_text_reserved(p->addr, p->addr))
         goto fail_with_jump_label;

     /* User can pass only KPROBE_FLAG_DISABLED to register_kprobe */
     p->flags &= KPROBE_FLAG_DISABLED;

     /*
      * Check if are we probing a module.
      */
     /*判断被探测的地址是否属于某一个模块，并且位于模块的text section内*/
     probed_mod = __module_text_address((unsigned long) p->addr);
     if (probed_mod) {
         /*如果被探测的为模块地址，首先要增加模块的引用计数*/
         /*
          * We must hold a refcount of the probed module while updating
          * its code to prohibit unexpected unloading.
          */
         if (unlikely(!try_module_get(probed_mod)))
             goto fail_with_jump_label;

         /*
          * If the module freed .init.text, we couldn't insert
          * kprobes in there.
          */
         /*如果被探测的地址位于模块的init地址段内，但该段代码区间已被释放，则直接退出*/
         if (within_module_init((unsigned long)p->addr, probed_mod) &&
          probed_mod->state != MODULE_STATE_COMING) {
             module_put(probed_mod);
             goto fail_with_jump_label;
         }
     }
     preempt_enable();
     jump_label_unlock();

     p->nmissed = 0;
     INIT_LIST_HEAD(&p->list);
     mutex_lock(&kprobe_mutex);

     jump_label_lock(); /* needed to call jump_label_text_reserved() */

     get_online_cpus();    /* For avoiding text_mutex deadlock. */
     mutex_lock(&text_mutex);

     /*判断在同一个探测点是否已经注册了其他的探测函数*/
     old_p = get_kprobe(p->addr);
     if (old_p) {
         /* Since this may unoptimize old_p, locking text_mutex. */
         /*如果已经存在注册过的kprobe，则将探测点的函数修改为aggr_pre_handler，并将所有的handler挂载到其链表上，由其负责所有handler函数的执行*/
         ret = register_aggr_kprobe(old_p, p);
         goto out;
     }

     /* 分配特定的内存地址用于保存原有的指令
      * 按照内核注释，被分配的地址必须must be on special executable page on x86.
      * 该地址被保存在kprobe->ainsn.insn
      */
     ret = arch_prepare_kprobe(p);
     if (ret)
         goto out;

     /*将kprobe加入到相应的hash表内*/
     INIT_HLIST_NODE(&p->hlist);
     hlist_add_head_rcu(&p->hlist,
          &kprobe_table[hash_ptr(p->addr, KPROBE_HASH_BITS)]);

     if (!kprobes_all_disarmed && !kprobe_disabled(p))
 /*将探测点的指令码修改为int 3指令*/
         __arm_kprobe(p);

     /* Try to optimize kprobe */
     try_to_optimize_kprobe(p);

 out:
     mutex_unlock(&text_mutex);
     put_online_cpus();
     jump_label_unlock();
     mutex_unlock(&kprobe_mutex);

     if (probed_mod)
         module_put(probed_mod);

     return ret;

 fail_with_jump_label:
     preempt_enable();
     jump_label_unlock();
     return -EINVAL;

     注册完毕，就开始kprobe的执行流程了。对于该探测点，由于其起始指令已经被修改为int3，因此在执行到该地址时，必然会触发3号中断向量的处理流程do_int3.

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 /* May run on IST stack. */
 dotraplinkage void __kprobes do_int3(struct pt_regs *regs, long error_code)
 {
 #ifdef CONFIG_KGDB_LOW_LEVEL_TRAP
     if (kgdb_ll_trap(DIE_INT3, "int3", regs, error_code, 3, SIGTRAP)
             == NOTIFY_STOP)
         return;
 #endif /* CONFIG_KGDB_LOW_LEVEL_TRAP */
 #ifdef CONFIG_KPROBES
     /*在这里以DIE_INT3，通知kprobe注册的通知链*/
     if (notify_die(DIE_INT3, "int3", regs, error_code, 3, SIGTRAP)
             == NOTIFY_STOP)
         return;
 #else
     if (notify_die(DIE_TRAP, "int3", regs, error_code, 3, SIGTRAP)
             == NOTIFY_STOP)
         return;
 #endif

     preempt_conditional_sti(regs);
     do_trap(3, SIGTRAP, "int3", regs, error_code, NULL);
     preempt_conditional_cli(regs);
 }

     在do_int3中触发kprobe注册的通知链函数，kprobe_exceptions_notify。由于kprobe以及jprobe等机制的处理核心都在此函数内，这里只针对kprobe的流程进行分析：进入函数的原因是DIE_INT3,并且是第一次进入该函数。

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 int __kprobes kprobe_exceptions_notify(struct notifier_block *self,
                  unsigned long val, void *data)
 {
     struct die_args *args = data;
     int ret = NOTIFY_DONE;

     if (args->regs && user_mode_vm(args->regs))
         return ret;

     switch (val) {
     case DIE_INT3:
 /*对于kprobe，进入kprobe_handle*/
         if (kprobe_handler(args->regs))
             ret = NOTIFY_STOP;
         break;
     case DIE_DEBUG:
         if (post_kprobe_handler(args->regs)) {
             /*
              * Reset the BS bit in dr6 (pointed by args->err) to
              * denote completion of processing
              */
             (*(unsigned long *)ERR_PTR(args->err)) &= ~DR_STEP;
             ret = NOTIFY_STOP;
         }
         break;
     case DIE_GPF:
         /*
          * To be potentially processing a kprobe fault and to
          * trust the result from kprobe_running(), we have
          * be non-preemptible.
          */
         if (!preemptible() && kprobe_running() &&
          kprobe_fault_handler(args->regs, args->trapnr))
             ret = NOTIFY_STOP;
         break;
     default:
         break;
     }
     return ret;
 }

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 static int __kprobes kprobe_handler(struct pt_regs *regs)
 {
     kprobe_opcode_t *addr;
     struct kprobe *p;
     struct kprobe_ctlblk *kcb;

     /*对于int 3中断，其被Intel定义为Trap，那么异常发生时EIP寄存器内指向的为异常指令的后一条指令*/
     addr = (kprobe_opcode_t *)(regs->ip - sizeof(kprobe_opcode_t));
     /*
      * We don't want to be preempted for the entire
      * duration of kprobe processing. We conditionally
      * re-enable preemption at the end of this function,
      * and also in reenter_kprobe() and setup_singlestep().
      */
     preempt_disable();

     kcb = get_kprobe_ctlblk();
     /*获取addr对应的kprobe*/
     p = get_kprobe(addr);

     if (p) {
 /*如果异常的进入是由kprobe导致，则进入reenter_kprobe(jprobe需要，到时候分析)*/
         if (kprobe_running()) {
             if (reenter_kprobe(p, regs, kcb))
                 return 1;
         } else {
             set_current_kprobe(p, regs, kcb);
             kcb->kprobe_status = KPROBE_HIT_ACTIVE;

             /*
              * If we have no pre-handler or it returned 0, we
              * continue with normal processing. If we have a
              * pre-handler and it returned non-zero, it prepped
              * for calling the break_handler below on re-entry
              * for jprobe processing, so get out doing nothing
              * more here.
              */
     /*执行在此地址上挂载的pre_handle函数*/
             if (!p->pre_handler || !p->pre_handler(p, regs))
 /*设置单步调试模式，为post_handle函数的执行做准备*/
                 setup_singlestep(p, regs, kcb, 0);
             return 1;
         }
     } else if (*addr != BREAKPOINT_INSTRUCTION) {
         /*
          * The breakpoint instruction was removed right
          * after we hit it. Another cpu has removed
          * either a probepoint or a debugger breakpoint
          * at this address. In either case, no further
          * handling of this interrupt is appropriate.
          * Back up over the (now missing) int3 and run
          * the original instruction.
          */
         regs->ip = (unsigned long)addr;
         preempt_enable_no_resched();
         return 1;
     } else if (kprobe_running()) {
         p = __this_cpu_read(current_kprobe);
         if (p->break_handler && p->break_handler(p, regs)) {
             setup_singlestep(p, regs, kcb, 0);
             return 1;
         }
     } /* else: not a kprobe fault; let the kernel handle it */

     preempt_enable_no_resched();
     return 0;
 }

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 static void __kprobes setup_singlestep(struct kprobe *p, struct pt_regs *regs,
                  struct kprobe_ctlblk *kcb, int reenter)
 {
     if (setup_detour_execution(p, regs, reenter))
         return;

 #if !defined(CONFIG_PREEMPT)
     if (p->ainsn.boostable == 1 && !p->post_handler) {
         /* Boost up -- we can execute copied instructions directly */
         if (!reenter)
             reset_current_kprobe();
         /*
          * Reentering boosted probe doesn't reset current_kprobe,
          * nor set current_kprobe, because it doesn't use single
          * stepping.
          */
         regs->ip = (unsigned long)p->ainsn.insn;
         preempt_enable_no_resched();
         return;
     }
 #endif
     /*jprobe*/
     if (reenter) {
         save_previous_kprobe(kcb);
         set_current_kprobe(p, regs, kcb);
         kcb->kprobe_status = KPROBE_REENTER;
     } else
         kcb->kprobe_status = KPROBE_HIT_SS;
     /* Prepare real single stepping */
     /*准备单步模式，设置EFLAGS的TF标志位，清楚IF标志位(禁止中断)*/
     clear_btf();
     regs->flags |= X86_EFLAGS_TF;
     regs->flags &= ~X86_EFLAGS_IF;
     /* single step inline if the instruction is an int3 */
     if (p->opcode == BREAKPOINT_INSTRUCTION)
         regs->ip = (unsigned long)p->addr;
     else
 /*设置异常返回的指令为保存的被探测点的指令*/
         regs->ip = (unsigned long)p->ainsn.insn;
 }

      对应kprobe,pre_handle的执行就结束了，按照代码，程序开始执行保存的被探测点的指令，由于开启了单步调试模式，执行完指令后会继续触发异常，这次的是do_debug异常处理流程。

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 dotraplinkage void __kprobes do_debug(struct pt_regs *regs, long error_code)
 {
     ....

     /*在do_debug中，以DIE_DEBUG再一次触发kprobe的通知链*/
     if (notify_die(DIE_DEBUG, "debug", regs, PTR_ERR(&dr6), error_code,
                             SIGTRAP) == NOTIFY_STOP)
         return;

     ....
     return;
 }

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 /*对于kprobe_exceptions_notify，其DIE_DEBUG处理流程*/
 case DIE_DEBUG:
         if (post_kprobe_handler(args->regs)) {
             /*
              * Reset the BS bit in dr6 (pointed by args->err) to
              * denote completion of processing
              */
             (*(unsigned long *)ERR_PTR(args->err)) &= ~DR_STEP;
             ret = NOTIFY_STOP;
         }
         break;

 static int __kprobes post_kprobe_handler(struct pt_regs *regs)
 {
     struct kprobe *cur = kprobe_running();
     struct kprobe_ctlblk *kcb = get_kprobe_ctlblk();

     if (!cur)
         return 0;

     /*设置异常返回的EIP为下一条需要执行的指令*/
     resume_execution(cur, regs, kcb);
     /*恢复异常执行前的EFLAGS*/
     regs->flags |= kcb->kprobe_saved_flags;

     /*执行post_handler函数*/
     if ((kcb->kprobe_status != KPROBE_REENTER) && cur->post_handler) {
         kcb->kprobe_status = KPROBE_HIT_SSDONE;
         cur->post_handler(cur, regs, 0);
     }

     /* Restore back the original saved kprobes variables and continue. */
     if (kcb->kprobe_status == KPROBE_REENTER) {
         restore_previous_kprobe(kcb);
         goto out;
     }
     reset_current_kprobe();
 out:
     preempt_enable_no_resched();

     /*
      * if somebody else is singlestepping across a probe point, flags
      * will have TF set, in which case, continue the remaining processing
      * of do_debug, as if this is not a probe hit.
      */
     if (regs->flags & X86_EFLAGS_TF)
         return 0;

     return 1;
 }

    至此，一个典型的kprobe的流程已经执行完毕了。

jprobe、kretprobe  to be continued...

    

参考链接：

1） http://www.ibm.com/developerworks/cn/linux/l-cn-systemtap1/index.html

2） http://blog.chinaunix.net/uid-23769728-id-3198044.html

3） http://zfpillar.devebar.net/2008/11/821.html

4）2.6.38 linux kernel（RTFC）